The present invention relates to an improvement of a temperature detection device for detecting the overheating of an electromagnetic clutch of an automobile or the like.
Generally, transmission of torque of an electromagnetic mechanism such as an electromagnetic clutch is on-off controlled by controlling power supplied to an exciting coil thereof. It is known that, due to a large friction loss caused by a difference in rotation number between an input shaft and an output shaft and the transmission torque, heat is generated in the clutch. Since the exciting coil is generally arranged in the vicinity of a portion of the clutch in which friction loss occurs, temperature of the coil may be increased, causing the electric resistance thereof to be increased which may lead to an over voltage or over current of an output transistor connected to the coil, damaging the latter as well as the output transistor. Therefore, in order to restrict heat generation to thereby prevent the output transistor and the coil from being damaged, it is usual to detect temperature of the coil to thereby control a current supply thereto. The temperature detection is performed by detecting a variation of voltage across the coil according to a coil voltage and a current instruction value.
FIG. 4 is a block diagram showing an example of a construction of a conventional control device of an electromagnetic clutch to be mounted on a car, which corresponds to FIG. 5 of Japanese Patent Kokai No. 57342/1988 which was invented by a co-inventor of this application.
In FIG. 4, the control device includes a clutch current control portion 1, a clutch current calculating portion 2 such as microcomputer and a car-battery 3. The clutch current control portion 1 includes a differential amplifier 14, a pulse width modulator (referred to as PWM hereinafter) 15 and a pair of output transistors 11 and 12. The clutch current calculating portion 2 is supplied with a car running control information SD and an engine control information SE. A clutch current instruction signal SI and a clutch open signal SO which are calculated by the clutch current calculating portion 2 on the basis of the control informations SD and SE are supplied to a (+) input terminal of a differential amplifier 14 of the clutch current control portion 1 and to a base of the output transistor 12, respectively.
An emitter of the transistor 12 is connected to a grounded negative terminal of the battery 3 through a clutch current detection resistor 13. The emitter of this transistor 12 is also connected to a (-) input terminal of the differential amplifier 14 to supply a current feedback signal SF thereto.
The differential amplifier 14 serves to obtain a difference between the clutch current instruction signal SI and the current feedback signal SF. An output of the differential amplifier 14 is connected to an input of the PWM 15 and an output of the latter is connected to a base of the transistor 11 so that the transistor 11 is on/off operated by an output signal of the PWM 15. An emitter of the transistor 11 is grounded through a parallel circuit of a diode 16 and a transistor 18 and is connected to an output terminal 20 of the clutch current control portion 1.
A collector of the transistor 12 is connected through a parallel circuit of a resistor 19 and an overvoltage preventing diode 17 to a positive terminal of the battery 3 and to another output terminal 21 of the clutch current control portion 1. Each of the resistors 18 and 19 is to supply a small reverse current to the electromagnetic clutch 4 when both the transistors 11 and 12 are turned off.
The electromagnetic clutch 4 is composed of a magnetizing coil 41 and the feeding mechanisms 42a and 42b connected in series with the coil. The output terminals 20 and 21 of the clutch current control portion 1 are connected to the feeding mechanisms 42a and 42b of the electromagnetic clutch 4, respectively, to supply an energizing current to the coil 41 when desired.
FIG. 5 is a circuit diagram showing the differential amplifier 14 and the PWM 15 of the clutch current control portion 1 in detail, which corresponds to FIG. 6 of the Japanese Patent Kokai No. 98822/1985 which was invented by a co-inventor of this application. In FIG. 5, similar portions to those in FIG. 4 are depicted by similar reference numerals and a dotted block depicted by 14, 15 shows the differential amplifier 14 and the PWM 15, in detail.
The clutch current instruction signal SI from the clutch current calculating portion 2 is supplied through a resistor 14a to the (+) input terminal of the differential amplifier 14. Resistors 14b and 14c are connected in series with each other between the (+) input terminal of the amplifier 14 and the positive terminal of the battery 3.
The PWM 15 is composed of resistors 15a and 15c and a transistor 15b.
The output terminal of the differential amplifier 14 is connected through the resistor 15a to a base of the transistor 15b and lo a junction between the resisters 14b and 14c.
The transistor 15b constitutes a main portion of the PWM 15 and has an emitter grounded and a collector connected through the resistor 15c to the base of the transistor 11.
A reference numeral 11a depicts a resistor connected between the emitter and the base of the transistor 11.
Although a circuit including the transistors 11 and 12 and the electromagnetic clutch 4 is somewhat different from that shown in FIG. 4, their functions are substantially the same.
To the base of the transistor 12, the clutch decoupling or open signal SO is supplied through a resistor 12a.
A reference numeral 50 depicts a voltage detection circuit which has a comparator 50a and a series circuit of a filter 24 and a zener diode 23 connected between the collector of the transistor 11 and ground.
The filter 24 is composed of a series circuit of a resistor 24a and a capacitor 24b and a junction of these elements is connected to a (+) input terminal of the comparator 50a. To a (-) input terminal of the comparator 50a, the clutch current instruction signal SI from the clutch current calculating portion 2 is supplied.
An output terminal of the comparator 50a is connected through a resistor 50b to the positive terminal of the battery 3 and to an output terminal 50c to provide a voltage detection signal SC2.
Now, an operation of the control device shown in FIGS. 4 and 5 will be described, starting from the clutch current calculating portion 2.
This clutch current calculation portion 2 operates to obtain a vehicle speed first and then an engine revolution. As mentioned the portion 2 responds to the running control information SD and the engine control information SE to calculate clutch torque in term of a clutch current.
When the electromagnetic clutch 4 is opened, the transistors 11 and 12 are turned on by the output of the PWM 15 and the open signal SO from the clutch current calculating portion 2.
As a result, a small reverse current flows through the resistor 19, the coil 41 of the electromagnetic clutch 14 and the resistor 18.
When the electromagnetic clutch 4 is connected, the transistor 12 is kept conductive and clutch current is detected through the clutch current detecting resistor 13.
The clutch current detected by the clutch current detecting resistor 13, that is, the current feedback signal SF is fed back to the (-) input terminal of the differential amplifier 14. The clutch current instruction signal SI from the clutch current calculating portion 2 is supplied to the (+) input terminal of this differential amplifier 14. The clutch current instruction signal SI and the current feedback signal SF are compared by the differential amplifier 14 and a resultant difference is supplied to the PWM 15. The PWM 15 pulse-width modulates the output of the differential amplifier 14 and supplies a resultant pulse to the base of the transistor 11. Therefore, this transistor 11 is on-off controlled according to the pulse width of the output of the PWM 15 to control the clutch current of the electromagnetic clutch 4.
Through the diode 16, a current circulates when the transistor 11 is turned off.
In FIG. 5, the temperature detection of the electromagnetic clutch 4 is performed during closure thereof which is caused by turning-on of the transistor 11. When the clutch 4 is closed a current flows through the coil 41 of the electromagnetic clutch 4 and a voltage drop across the electromagnetic clutch 4 including the feeding mechanisms 42a and 42b is applied to the filter 24. The filter 24 provides a temporal coil resistance due to the voltage drop caused by the pulse-width modulated, magnetizing current which flows through the magnetizing coil 41.
Voltage drops of the feeding mechanisms 42a and 42b are substantially constant regardless of the temperature of the clutch and current flowing therethrough and, therefore, the transistor 12 and the diode 16 are influenced directly by a forward component of the voltage drop. Therefore, for the forward drop of the zener diode 23, voltage drops of the feeding mechanism 41, 42 and the negative said transistor 12 can be cancelled.
That is, in FIG. 5, the comparator 50a compares the voltage drop across the electromagnetic clutch 4 supplied from the filter 24 with the clutch current instruction signal SI and provides a detection signal SC2. In other words, the temperature of the electromagnetic clutch 4 is detected by increase and decrease of clutch resistance.
Since the electromagnetic clutch 4 includes the feeding mechanisms 42a and 42b and the current feedback signal SF contains the voltage drops of the transistor 12 and the clutch current detection resistor 13, this current feedback signal SF contains substantial error.
Further, the output of the clutch current control portion 1 is a pulse-width modulated, on/off signal having magnitude corresponding to the source voltage. An overvoltage of the electromagnetic clutch 4 is detected by the voltage detection circuit 50 with an average voltage obtained by averaging this output voltage by the filter 24.
Therefore, it is impossible to obtain high precision of voltage detection and thus it is impossible to obtain accurate temperature detection. As a result, when used in a vehicle it is inadequate to improve the commercial quality of the product and the safety thereof.
Further, since temperature is detected by increase and decrease of the clutch resistance according to variations of the clutch current instruction signal SI and the current feedback signal SF, as mentioned above, when the source voltage saturates, that is, when an error between the clutch current instruction signal SI and an actual clutch current is large, it is impossible to detect temperature exactly and thus it is inadequate for use in a vehicle.